The present invention relates to a die for injection molding used in injection molding and able to manufacture a tape cassette, a disk cartridge, etc. formed by synthetic resin and other molding materials, with high precision and good productivity, and also relates to an injection molding method using this die. More particularly, the present invention relates to a die for injection molding which can promote cooling and solidification of a molded product by holding this molded product in a cavity block as long as possible at a die opening time and can obtain a stable molded product by preventing its deformation at a taking-out time of the molded product, and also relates to an injection molding method using this die.
In general, a box body of a tape cassette, a disk cartridge, etc., is constructed by a pair of shells (also called halves) overlapped with each other and uses thermoplastic resin as its material. Many molded products are simultaneously manufactured by injection molding using a die for injection molding constructed by a fixed side die and a movable side die and able to be opened and closed. Schematic processes of such injection molding are provided as follows.
The die is first clamped (the die clamp is held until the die is opened after a cooling time), and a nozzle next comes in contact with the die by advancing an injecting unit. A molding material is then injected and a die cavity is filled with this molding material. Next, shrinkage of the molding material is restrained by applying a holding pressure to this molding material. Further, a screw is rotated and the next molding material is supplied into a heating cylinder. Next, the nozzle is retreated. When the molding material is cooled and solidified, the die is opened and a molded product is projected out of the die by ejector pins so that one cycle of an injection molding process is completed. The injection molding is continuously performed by repeating this process.
For example, a conventional die for injection molding used in such injection molding is provided as shown in FIGS. 1 to 3. This die for injection molding is constructed by a fixed side die 1 and a movable side die 2. The fixed side die 1 has a fixed side attaching plate 3, a fixed side receiving plate 4, a fixed side die plate 5 and a cavity block 6. The movable side die 2 has a movable side attaching plate 7, a spacer plate 8, a movable side receiving plate 9, a movable side die plate 10 and a core block 11
Further, a sprue bush 12 and a locating ring 13 are arranged in the fixed side attaching plate 3 of the fixed side die 1. An injection molding nozzle of an injection molding machine is fitted into the sprue bush 12 and the locating ring 13. A gate of a sprue 14 at its end tip extending from the sprue bush 12 to the cavity block 6 is opened to a cavity 15 of the cavity block 6.
A projecting plate 16 is arranged on a rear face of the movable side receiving plate 9 of the movable side die 2. A plurality of kinds of ejector pins 17, 18 have base portions fixed to this projecting plate 16 and extend through the movable side receiving plate 9 and the core block 11. Similarly, a guide pin 19 has a base portion fixed to the projecting plate 16 and extends through the movable side receiving plate 9 and the movable side die plate 10. An end tip face of the one ejector pin 17 faces the cavity 15. A core 20 is fixed to an end tip of the other ejector pin 18. An end tip face of the core 20 faces the cavity 15. Further, a return spring 21 is externally fitted onto the guide pin 19. The projecting plate 16 is biased by resilient force of the return spring 21 on a side of the movable side attaching plate 7.
Further, an ejector block 22 is fixed to the projecting plate 16. The projecting plate 16 is pushed forward by a projecting rod 23 of the injection molding machine facing this ejector block 22 so that the ejector pins 17, 18 and the guide pin 19 are respectively projected onto a side of the cavity 15.
Thus, as shown in FIG. 1, melted resin injected from the injection molding nozzle of the injection molding machine is injected into the cavity 15 from the sprue gate through the sprue 14 held at a high temperature. After the melted resin is cooled and solidified within the cavity 15, as shown in FIG. 2, a molded product S is separated from a face of the cavity 15 by opening the die. Thereafter, as shown in FIG. 3, the projecting plate 16 is pressed by the projecting rod 23 so that the ejector pins 17 and 18 are projected out. Thus, the molded product S is separated from a face of the core block 11 and is taken out of this face.
However, in the above-mentioned die for injection molding, as shown in FIG. 2, at a die opening time in the taking-out of the molded product, the molded product S is attached to the core block 11 and is moved integrally with this core block 11 simultaneously when the die is opened. Therefore, an external face of the molded product is pulled by adhesive force between the molded product and the cavity 15 so that the molded product S is deformed in many cases. Further, as shown in FIG. 3, when the molded product S is projected, the molded product S is pressed and separated by the ejector pins 17 and 18 against adhesive force between the molded product S and the core block 11. Therefore, the molded product S is similarly deformed by resisting force caused in this separation in many cases. Accordingly, no stable molded product can be obtained in any case.
Specifically, as shown in FIG. 1, the cavity 15 of the die is filled with the melted resin by injecting this melted resin. The melted resin is then solidified by cooling the melted resin in a pressure holding state. However, a considerable time is required until this melted resin is completely solidified. Therefore, when the die is opened after the complete solidification of the melted resin, a considerable time is taken until the die is opened, thereby reducing production efficiency. Therefore, in general injection molding, the die begins to be opened so as to increase productivity of the molded product S of this kind when the melted resin within the cavity 15 is solidified to a certain extent even before this melted resin is completely solidified.
This die opening operation is performed by retreating the movable side die 2. When the movable side die 2 is retreated, the molded product S is retreated integrally with the core block 11 since adhesive force between the molded product S and the core block 11 is generally stronger than that between the molded product S and the face of the cavity 15. Thus, an external face of the molded product S is separated from the cavity face. However, when the external face of the molded product S is separated from the cavity face, separating force is applied to the molded product S by resistance (hereinafter, called "mold-releasing resistance") caused in the die separation based on e.g., an extraction gradient of the cavity 15, a shape of the cavity 15 itself, etc. Therefore, as shown in FIG. 2, a peripheral edge portion of the molded product S is pulled onto a side of the cavity block 6 and is deformed in many cases.
Further, as shown in FIG. 3, when a projecting operation of the molded product S is started before the resin of the molded product S is completely solidified, the molded product S is deformed by mold-releasing resistance caused on the basis of e.g., an extraction gradient of the core 20, a shape of the core 20 itself, etc., and the molded product S is scattered by a sudden separating operation in many cases.